As a man becomes older he needs to be fit and take some exercise, or his ability to "perform" goes away. Being overweight and having clogged-up arteries make it even worse.
The ability - and the desire - also depends on having more testosterone and less estrogen.
If these are right he will have sexual thoughts like a teenager, wake with an erection and be chasing you round the bedroom !
If they are wrong, he will lose interest in sex, have erectile dysfunction and avoid exposing the problem
The whole subject is so emotionally charged that it is very difficult to deal with
Most doctors don't have the time to deal with all these aspects of the problem, so I have some totally different suggestions:
1. Search online for "BANT" ( British Association of Nutritional Therapists )
Search for 10 therapists near you and contact all of them discretely to see which ones have dealt with this many times before and have a good track record
Whittle that down to 3 and then go and meet with them - on your own - before committing to a consultation
I suggest this because changing his diet may be the most significant thing that you can do
2. Re-create the intimacy that you may once have had, with no suggestion of it leading to sex. By that I mean - be best friends, hold hands, snuggle up to him, kiss him and tell him that you love him, and no negative vibes
3. Take him by the hand and go for walks together
4. Take mini breaks, like long weekends away where you share you innermost thoughts and dreams and even fantasies
The end result is that you help him to eat more healthily, lose weight and take some exercise - and you fancy each other more !
Worth a try ?